The invention pertains to a process and system for optically inspecting transparent containers, such as plastic and glass beverages containers.
According to DE 3,532,068 C, several bottles a certain space apart are simultaneously inspected optically as they are being conveyed continuously through an inspection area. Each bottle is rotated at least 360xc2x0 around its longitudinal axis within the inspection area. A single video camera takes partial pictures of the side walls of the bottles at a certain rate and combines them into a single image for each bottle. These images are then examined in an evaluation unit to determine the presence of damage or dirt. The stationary illuminating unit has a light source with a diffuser in front of it. Because the intensity of the light at the two-dimensional opening of the diffuser is the same for all of the bottles passing in front of it and because the bottles can have different degrees of transparency as a result the material of which they are made and/or the wear to which they have been subjected, the brightness of the images produced necessarily varies also, which considerably impairs the quality of the inspection results. Overexposed images are almost impossible to evaluate.
In practice, LED screens are now also being used as illuminating units for the optical inspection of at least partially transparent containers. The LEDs of the light screen are activated jointly and produce light of a certain intensity. An upline measuring device determines the degree of transparency of each container, and the light intensity is adjusted if necessary. If the degrees of transparency of the individual containers differ significantly from each other, however, this adjustment represents only a compromise applicable to the few containers passing through the inspection area.
The invention is based on the task of providing a process of the general type indicated above and a system suitable for implementing the process, by means of which the containers can be inspected with uniformly high quality regardless of variations in their degrees of transparency.
The task thus defined is accomplished with respect to the process by the features of claim 1 and with respect to the system by the features of claim 8.
According to the process, a traveling light field, which is configured for each individual container, travels along with the container. As a result, each container can be inspected more effectively than it can be with a stationary light field, which is the same for all of the containers being inspected simultaneously. By adapting either the light intensity of the light field on the illumination side and/or the imaging sensitivity on the imaging side to the individual transparency of the container while the container is moving together with its own light field, variations in the degrees of transparency can be compensated in such a way that the image used for the evaluation of one container will be essentially equal in brightness to the image used for the evaluation of another container with a different degree of transparency. As a result, the influence of the individual degree of transparency on the quality of the evaluation is eliminated, and the accuracy of the inspection remains on a uniformly high level.
Because the LED light screen in the device has LEDs which can be activated either individually or in groups, it is possible for a control unit to configure the light field so that it is adapted to each container to be inspected, with this light field then traveling synchronously with the container through the inspection area. The measuring unit determines the individual degree of transparency of each container. Under consideration of the measurement results, the light intensity in the traveling light field can be adjusted so that variations in the degrees of transparency between containers are compensated. This leads to the situation that the camera used for inspection produces images of equal brightness for all containers, regardless of their individual degrees of transparency. There are no longer any significant differences in brightness between the images. Dirt or damage is thus detected with uniformly high quality. Because the individually activated or group-activated LEDs configure a different light field for each container in front of the light screen and because this field travels along with the container, it is easy for several containers to be inspected simultaneously, even though the illumination is adapted individually to each one. The device is especially suitable for the inspection of upright bottles of plastic or glass, the light being either transmitted through the side walls of the container or incident upon them. The bottles can also be rotated around their longitudinal axes during the inspection process if desired. This does not exclude the possibility that the bottom surfaces of the containers or the areas around the mouth are also inspected in the same way. The brightness of the images is selected so that even opaque layers of dirt are recognized, whereas no containers are rejected in error as a result of insufficient brightness.
Because variations in the degrees of transparency are compensated on the illumination side and/or on the receiving side, at least two or more containers traveling in the transport direction a certain distance apart can be inspected simultaneously in the inspection area on a high level of inspection quality.
In a simple process variant, the light intensity in the traveling light field and/or the imaging sensitivity over the height of the image is adjusted to a uniform value for the individual overall degree of transparency of the container. This alone is enough to provide high inspection quality in practical applications in which the degrees of transparency of the containers do not fluctuate significantly in the height direction.
It can also be advisable, however, to determine the variations in the transparency of each container in the height direction and to adapt the light intensity and/or the imaging sensitivity in correspondence with these detected variations.
Under certain conditions, variations in the degree of transparency in the transport direction are also determined and equalized on the illumination and/or on the reception side.
The concomitant motion of the traveling light field and its associated container and the adjustment of the light intensity of this traveling light field to the individual degree of transparency of the container can be easily implemented in terms of the process technology involved by means of an LED light screen consisting of a plurality of LEDs, which can be activated either individually or in groups. It is advisable for the LEDs to be activated in rows parallel to the axis of the container. It is especially favorable for the illumination to be continuous.
The adaptation of the light intensity in the traveling light field for the purpose of equalizing the variations in the degree of transparency or for adapting the intensity to the individual degree of transparency can be easily accomplished in terms of the process technology involved by adjusting the current strength and/or the flash time and/or the number or distribution of the activated LEDs.
To increase the inspection quality even more, it can be advisable to vary the light intensity and/or the imaging sensitivity also as a function of the transport position and/or the rotational position of each container in the inspection area. For example, the actual distance between the container and the camera can be a parameter of such variation, or, in the case of a container which is not round, the rotational position during imaging in relationship to the camera can be used as a parameter. The information required to take these parameters into account can be easily defined in terms of the process technology involved by appropriate sensors or stored programs.
With respect to the system, the control device is designed in such a way that it configures the light field on the LED light screen for each container and moves it through the inspection area together with the container. The compensation device uses the information it receives from the measuring device to adjust the light intensity in the traveling light field accordingly. The control device and the compensation device work together to ensure that the images are uniformly bright regardless of the variations in the degrees of transparency.
It is advisable for the control device and the compensation device to be combined into a common electronic control unit, to which the measurement results of the measuring device and, for example, the position and velocity of each container in the inspection area are transmitted in the form of signals.
If the automatic control is based on an average degree of transparency, a simple measuring device with essentially only a single light source and a measurement signal receiver is all that is required.
In the case that the transparency profile of the container is measured and factored into the adjustment of the light intensity or imaging sensitivity, a measuring device in the form of a linear measuring array with several measuring cells and measuring sections and possibly several light sources can be favorable. To obtain this profile information, different brightnesses in the measuring cells, for example, or different light intensities of the light sources required to achieve equal brightnesses at the measuring cells are taken into account.
In a simple embodiment, the LEDs in the LED light screen can be activated in rows parallel to the axis of the container. The traveling light field can be configured by one or more rows; all the LEDs in one row do not necessarily have to be activated.
The light intensity of the traveling light field can be adjusted to the measured degree of transparency or to the transparency profile by making the appropriate changes to the current strength and/or the flash time. It is also possible, however, to vary the number and/or the distribution of the activated LEDs.
Because of the demand for uniformly high inspection quality and because of the high container transport velocities normally used in modem machines, it is advisable for at least one microprocessor with appropriate programmability to be present in the control unit.
The microprocessor can have a program memory section, where parameters used repeatedly during the adjustment of the light intensity and/or of the imaging sensitivity can be stored for a certain type of container. This is an especially effective measure in cases where a simple measuring device is provided to determine merely an average degree of transparency and where an average container transparency profile is known.
The device is especially suitable for the inspection of the side walls of glass or plastic bottles according to the transmitted light or incident light principle. It is also conceivable, however, that the bottoms or mouths of the bottles could also be inspected.